1. Field of the Invention
The present invention is directed to measurement of the polarization of light and the polarization properties of samples.
2. Description of the Related Art
The polarization state of light can be measured by taking N measurements of intensity in time-series, while one or more polarizers or waveplates are stepped through a sequence of settings. From the intensities obtained at each of the N settings, the polarization state of incident light is determined. Various arrangements of polarizers and waveplates have been devised for use in determining the Stokes vectors for light of arbitrary polarization.
One may use such an instrument to determine the birefringence or retardance of a sample by illuminating the sample with light of a known polarization state, and measuring the polarization state of light exiting the sample; from the difference between the incident and exit states of polarization, the optical properties of the sample are inferred.
Systems have been devised for imaging measurements of samples with low retardance, such as optical glasses, biological tissue, and cells including oocytes, using imaging detectors such as CCD cameras. Oldenbourg and Mei describe such a system in U.S. Pat. No. 5,521,705 that uses four images to determine retardance magnitude and slow axis orientation angle for a sample that is illuminated with circularly polarized light.
Systems that require taking several exposures in series cannot operate in real-time, and since most utilize video-rate detectors, the total elapsed time can be significant. Further, if there is sample movement during the measurement sequence, inaccurate or unreliable results can be obtained.
So-called instantaneous polarimeter systems have been devised for stress-measurement of samples that exhibit retardances of λ/4 or more. Such systems divide a beam of light from the sample into N sub-beams and perform the required N measurements in parallel, to provide a real-time measurement of polarization state. In some of these, an image is obtained, such as the instrument described in U.S. Pat. No. 6,055,053. This system utilizes partial reflection at beamsplitter elements to divide the beams, which are then delivered to multiple CCD detectors. Another instantaneous polarimeter is described in U.S. Pat. No. 6,441,972, which produces the multiple images at a single CCD detector for easier construction.
The optical system of U.S. Pat. No. 6,441,972 utilizes a plurality of sectored lens slices or a plurality of prisms to produce multiple images at an imaging detector. The prisms are thinner in the center of the beam, to deflect multiple copies of the image outward, into separate images. Alternatively, the lens slices are shifted radially outward for the same purpose (with an attendent loss of light due to the missing space between lens slices). Beyond the performance issues involved, such a system is less than optimal due to its complexity: it requires the fabrication of, and then precise registration of, many lens slices or prisms.
The fruit-grading apparatus of Blit et al.(U.S. Pat. No. 5,526,119) uses a multifaceted prism to create multiple images of a sample. In this system, each prism face is treated with an optical coating so that several images are produced simultaneously at a common CCD detector, each corresponding to a different spectral band. No measurement of polarization is provided by this apparatus.
Retardance imaging has been used to a limited degree in cellular biology research, and in in-vitro fertilization of oocytes, using sequential measurement systems with N=4. But the low measurement speed of such systems has been a barrier to its wider acceptance in these fields.
Overall, no instrument provides the ability to image low-retardance samples in real-time, or nearly so. Such samples may present only a few nanometers of retardance, and for useful imaging, a noise level of 0.2 nm or less in the computed retardance image is beneficial.
It is a goal of the present invention to provide an instrument for measurement of polarization state or sample retardance, which provides greatly improved measurement speed without compromising the accuracy and sensitivity of the readings obtained for low-retardance samples. It is a further goal to provide an apparatus that yields full two-dimensional images of low-retardance biological, medical, and industrial samples in near real-time. Another aim of this invention is to provide these benefits without need for complicated or expensive optical elements. A further goal is to provide designs that greatly reduce polarization errors in the instrument, so the retardance or polarization signals are of high quality. Yet another goal is to provide methods for calibrating and removing any residual errors by data reduction algorithms, so that a high sensitivity and a low noise floor are obtained, comparable to what is achieved by time-sequential measurements.